Pain as a teacher

Pain is a good teacher.

When a child burns himself, he learns that fire is hot and can hurt. When a child falls off the table, he learns that high places are dangerous. When you didn't catch your teacher's instructions and have to rewrite your assignment, you learn the importance of paying attention. When you shout back at a customer and lose your job, you learn the importance of patience. In Kingdom of Heaven, after taking his oath of knighthood, Balian was slapped by father, so that he remembers his oath. Pain as a teacher to help him remember his oath.

Pain is a good (effective) teacher, for we all remember the lessons most painful.

But pain is not the only teacher. That, we must always remember. There are more than one way to teach someone lessons that he will remember.